Vitrinite Reflectance Study of the Upper Cretaceous Fruitland Formation, San Juan Basin, New Mexico

PAWLEWICZ, MARK J., and VITO F. NUCCIO, U.S. Geological Survey, Denver, CO

In 1988, a cooperative project between the New Mexico Oil Conservation Division and the U.S. Geological Survey was established to better understand the thermal maturity of coal beds in the Upper Cretaceous Fruitland Formation throughout the New Mexico portion of the San Juan basin. The first product of this project was U.S. Geological Survey Open-File Report 90-290, which presented vitrinite reflectance values for 125 coal samples. More recently, 300 additional vitrinite reflectance samples from wells throughout the area have been analyzed. Vitrinite isoreflectance maps, structure contour maps, and cross sections, which combine the old and new data, were constructed to illustrate thermal maturation patterns of the Fruitland Formation across the San Juan basin in New Mexico.

Vitrinite reflectance (Rm) values for the Fruitland Formation increase from south to north and generally follow the structural configuration of the basin. This pattern is directly related to depth of burial in the basin and indicates that maximum maturity (0.40% along the southern margin of the basin to 1.30% near the New Mexico-Colorado border) was established prior to final uplift. Because of the high density of data, local anomalies in the general pattern can be delineated. For instance, samples from a group of wells in T30N, R8W have Rm values that average 0.80% at depths of around 3100 ft (945 m), whereas in T32N, R7W, only 10 mi away, the Rm values average 1.20% at similar depths of about 3300 ft (1000 m). These anomalies may be explained by localized areas of high heat flow fro deep heat sources or by migration of relatively hot fluids along fracture and fault systems.

In the New Mexico part of the San Juan basin, Fruitland Formation coals produce methane at maturities corresponding to Rm values between 0.60% and about 1.30% at depths of approximately 3000 to 4000 ft (915 to 1220 m), respectively. Using our vitrinite reflectance data, we have mapped areas of optimum potential for coal-bed methane production in the New Mexico portion of the San Juan basin that generally include the areas between R4-10W and from T27N northward to the New Mexico-Colorado border.

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)